Project Summary Hijacked plasticity of cell fate potential, combined with oncogenic mutations, drives malignant transformation and therapeutic resistance in many cancers. Totipotency ? the ability of a cell to become all embryonic and extra- embryonic tissues ? represents the height of embryonic/stem cell fate potential in mammals. DUX is a double homeodomain transcription factor that is misexpressed in subsets of B-cell acute lymphoblastic leukemia and round-cell sarcomas. Our lab recently defined DUX (mouse Dux / human DUX4) as a major driver of totipotent developmental programs in mouse and human. Pluripotent mouse embryonic stem cells (mESCs) expressing mouse Dux are potently converted into a ?2C-like? totipotent state, which epigenetically and transcriptionally resemble the 2-cell mouse embryo. In vivo, DUX expression is highly transient, and lasts less than a cell cycle in early embryos. Although the transcriptome of totipotency is well studied, it is unclear how the embryo employs post-transcriptional and translational regulation downstream of DUX to establish this plasticity in cell fate, and what mechanisms subsequently restrict developmental potency to ensure proper lineage determination. Recently, we and others have discovered a central role for p53 in DUX activation. P53 is a master tumor suppressor and transcription factor, which also activates the micro-RNA (miRNA) 34 family (a/b/c) in response to DNA damage. miR-34a is expressed in many mouse and human tissues, and is a well-studied tumor suppressor itself, repressing pathways involved in cellular growth and proliferation. Importantly, miR-34a deletion in mESCs confers a high probability of their conversion into a totipotent state, however the mechanisms governing this expanded cell fate plasticity are unclear. We find that loss of miR-34a in mESCs causes accumulation of Dux transcripts, and DUX4 contains a predicted miR-34a target site in its 3? UTR, suggesting a direct regulatory role of miR-34a. We hypothesize that p53 and miR-34a activate and repress Dux (and/or Dux targets), respectively, forming a feedback loop which regulates the totipotency network and ultimately restricts cell fate plasticity in vivo. Uncovering the mechanisms safe-guarding cell fate potential and preventing oncogenesis in vivo will yield novel insights in both cancer biology and regenerative medicine. I am uniquely positioned to answer these questions in the Cairns Lab within the Huntsman Cancer Institute at the University of Utah. Leveraging our advanced DUX expertise, combined with key collaborations with experts in translational control, I am confident in my abilities to execute the experiments outlined within this proposal. Additionally, my excellent clinical mentorship team, crafted from highly productive physician- and surgeon-scientists, will promote my clinical development as I seek to combine key insights from molecular drivers of cell fate plasticity and oncogenesis into development of novel targets for cancer therapeutics and regenerative medicine.